Pairwise model discrimination with applications in lifetime distributions and degradation processes

Reliability data obtained from life tests and degradation tests have been extensively used for purposes such as estimating product reliability and predicting warranty costs. When there is more than one candidate model, an important task is to discriminate between the models. In the literature, the model discrimination was often treated as a hypothesis test and a pairwise model discrimination procedure was carried out. Because the null distribution of the test statistic is unavailable in most cases, the large sample approximation and the bootstrap were frequently used to find the acceptance region of the test. Although these two methods are asymptotically accurate, their performance in terms of size and power is not satisfactory in small sample size. To enhance the small‐sample performance, we propose a new method to approximate the null distribution, which builds on the idea of generalized pivots. Conventionally, the generalized pivots were often used for interval estimation of a certain parameter or function of parameters in presence of nuisance parameters. In this study, we further extend the idea of generalized pivots to find the acceptance region of the model discrimination test. Through extensive simulations, we show that the proposed method performs better than the existing methods in discriminating between two lifetime distributions or two degradation models over a wide range of sample sizes. Two real examples are used to illustrate the proposed methods.     

Some new results on the preservation of the NBUE and NWUE aging classes under the formation of coherent systems

We consider the classical problem of whether certain classes of lifetime distributions are preserved under the formation of coherent systems. Under the assumption of independent and identically distributed (i.i.d.) component lifetimes, we consider the NBUE (new better than used in expectation) and NWUE (new worse than used in expectation) classes. First, a necessary condition for a coherent system to preserve the NBUE class is given. Sufficient conditions are then obtained for systems satisfying this necessary condition. The sufficient conditions are satisfied for a collection of systems which includes all parallel systems, but the collection is shown to be strictly larger. We also prove that no coherent system preserves the NWUE class. As byproducts of our study, we obtain the following results for the case of i.i.d. component lifetimes: (a) the DFR (decreasing failure rate) class is preserved by no coherent systems other than series systems, and (b) the IMRL (increasing mean residual life) class is not preserved by any coherent systems. Generalizations to the case of dependent component lifetimes are briefly discussed.     

Unsolved murders and the investigative failures of the Nigerian police force: Security and sociopolitical implications

This article focuses on the systemic failures of the Nigerian police force to unravel the mysteries surrounding homicides in the country, along with the security, social and political implications of the ongoing trend. The article draws on documented pieces of evidence of high-profile murders, along with the causes of the upsurge in murder incidences and the inhibitive factors relating to the investigative procedures of the police. The suitability of the theory of subaltern realism and the concept of third-world security predicaments in explaining the peculiarities of Nigeria’s internal security challenges is examined, along with the problems of the legitimacy of political regimes and weak borders. There is a need for the National Assembly of Nigeria to exercise its constituted powers in reawakening and supervising investigations into dormant cases of murder through its committees on police affairs, public safety and national security. The police should check its organisational failures, which may be related to using the wrong investigative approaches, low levels of information, limited resources, corrupt practices, and pressure to obtain quick results.     

基于平衡计分卡的武器装备维修绩效评价指标体系构建

武器装备维修是武器装备维修保障的重要内容,加强装备维修绩效评价,建立科学规范的绩效评价指标体系,对深入推进装备维修工作健康发展将发挥积极作用。以平衡计分卡为研究手段,运用"5M1E"分析法,从一个内核和四个维度出发,分析武器装备维修绩效评价的影响因素,据此建立评价指标体系,并应用于装备维修管理实践,为有效提高装备维修保障效益提供有益借鉴。     

关于军民融合大交通体系建设的几点思考

党的十九大报告把军民融合发展战略作为七大国家战略之一,明确提出要"更加注重军民融合"。必须将交通领域军民融合视作整个军民融合深度发展的重要突破口,建设军民融合大交通体系。提出以新形势下军事战略方针为统揽,以军民深度融合为途径,聚焦保障军队打赢,大力推进民用交通设施工具贯彻要求;依托国家路网基础,大力推进军地交通运输网络统筹纳入;强化应战应急一体,大力推进各种交通力量建设互补衔接;着眼战略投送能力,大力推进军民交通物流体系有机结合。     

星载大容量固态存储控制器的级联编码设计

航天器大容量数据存储设备主要采用基于NAND Flash的固态存储器,但由于空间环境中单粒子翻转效应的影响,以及存储器芯片在操作过程中因为阈值电压偏移导致的位比特错误等原因,存储设备的可靠性降低。为提高数据存储设备的数据容错性,依据NAND Flash芯片物理结构和数据存储结构,具有针对性地提出RS(256,252)码+LDPC(8192,7154)码级联的纠检错并行编码设计,并优化编码算法的电路实现方法。建模仿真和地面测试系统测试结果表明:该设计具有低硬件开销、低功耗和高可靠性的优点。存储系统的数据总容量达512 Gb,有效数据吞吐率为700 Mb/s,能够满足航天器固态存储控制器对大容量数据控制和高数据吞吐量的设计需求。     

重塑军队作战力量体系的几点思考

作战力量体系是一个国家军事力量体系的核心组成部分。近些年通过深化国防和军队改革,我军军事力量体系宏观上发生了脱胎换骨的变化,接下来到了对作战力量体系进行重塑的攻坚阶段,本文结合当前作战力量体系发展的一些特点和建设存在的短板,提出了重塑我军作战力量体系的决策建议。      

火箭-双燃烧室冲压组合循环发动机概念研究

临近空间高超声速飞行器近年来获得了广泛关注,本文提出一种以基于火箭发动机和双燃烧室冲压发动机的多模态火箭-双燃烧室冲压组合循环发动机作为飞行器的动力系统,并进行了性能分析研究。该飞行器在海拔10 km左右高度以0.8马赫的速度投放,在重力和发动机推力的联合作用下,能够在海拔5～8 km处加速到2马赫;然后加速爬升进入临近空间,发动机工作在引射亚燃或者双燃烧室亚燃模态下。可以根据实际选择高推重比、较低推进剂比冲效率的引射亚燃模态,或是较低推重比、高推进剂比冲效率的双燃烧室亚燃模态。最终飞行器加速到6马赫(26 km),进入双燃室超燃模态。针对空中发射模式和地面发射模式进行了轨道优化,仿真结果表明:在加速爬升到6马赫(26 km)的过程中,空中发射模式相比较地面发射模式可以节省37%的推进剂;空中发射模式存在一个负的最优初始飞行角度使得剩余质量与初始质量的比值达到最大。     

指数寿命产品可靠性增长试验的Bayes分析

针对指数寿命产品的定时、定数截尾试验方案,推广了Mazzuchi-Soyer模型的应用范围。首先引入模型假设,以狄氏分布作为先验分布,综合利用产品研制的历史信息和专家信息,结合产品研制各阶段试验数据,给出了各阶段可靠性的联合后验分布。然后利用Gibbs抽样算法解决后验推断计算问题,得到各阶段产品可靠性的Bayes点估计和区间估计。最后给出产品可靠性增长分析实例,表明了模型的优越性。     

小子样条件下基于Normal-Poisson过程的性能可靠性评定

对于高可靠、长寿命产品,基于性能退化数据分析可靠性是一种行之有效的技术途径。结合航空航天产品性能退化的机理与现场试验小子样的特点,建立了基于Normal-Poisson过程的性能退化模型。论文在对产品性能退化建模的基础上,结合Bayes方法给出了退化模型参数的估计算法和可靠性推断的公式,最后结合实例说明了方法的有效性。